1. Field of the Invention
The present invention generally relates to restoration strategies in a packet transfer network composed of a plurality of routers, and in particular to a method and system for restoring data transmission when a fault occurs on a transmission line between routers.
2. Description of the Related Art
There have been proposed restoration strategies for a packet transfer network composed of a plurality of routers, typically, the Internet. An example of a basic restoration method is to switch an initial route to a restored route when a failure has been detected between routers in the initial route. Such a basic restoration method needs to find a new route as an alternative route at its endpoint router by searching its routing table which is being updated according to the routing protocol. Accordingly, it will take much longer until restoration. It is difficult to put it in practical use especially in the field of real-time data transmission such as voice and video data transmission.
As another example of restoration technique, a router network having a proxy router has been disclosed in Japanese Patent Application Unexamined Publication No. 11-261620. The network environment of a LAN belonging to a router is previously copied to another router When a failure occurs at the router, the other router serves as a proxy router of the failed router so that communications of the LAN belonging to the failed router can be maintained.
It is possible to apply the line protection technique employed in SONET/SDH (Synchronous Optical NETwork/Synchronous Digital Hierarchy) to the packet transfer network. More specifically, the same signal is transferred through both of working and protection lines. In case of working line cut or node failure, the same signal on the protection line can be transmitted to a destination node. However, all information are transferred through both working and protection line independently of its degree of importance, resulting in reduced efficiency on the use of the link bandwidth.
Recently, to protect connections for packet communications, there has been proposed a protection and restoration strategy using a 1+1 protection method and a packet flow selector in Japanese Patent Application No. 10-349220 filed on Dec. 9, 1998 (Unexamined Publication No. P2000-174815A published on Jun. 23, 2000). According to this strategy, when a packet carries important information, the packet is duplicated and the duplicate packets are transferred through different routes. At the receiving node, the first arrival of the packet is used and the other duplicate packet is discarded. Such a strategy can avoid causing important information to be lost due to link failure.
However, the duplicate important packets flow through the network at all times. Therefore, the greater the amount of important information, the greater the amount of packet flow to be protected. This causes the efficiency on the use of the network bandwidth to be reduced.